As a background art of this technical field, JP-A-2008-304407 (Patent Literature 1) is disclosed. In this publication, disclosed is “a reflective surface of a prism is fixed on the axial line of a laser beam emitted from a laser displacement meter. Thus, even when a measuring object is changed or the shape of the inner peripheral surface to be measured is varied, an operation to adjust arrangement of the laser displacement meter and the prism is unnecessary, thereby allowing measurement of the inner diameter D with a simple operation. The reflective surface of the prism is arranged so as to protrude from a support, even when the diameter of the inner peripheral surface of the measuring object is small, and the entire measurement part cannot be guided into the inner peripheral surface, the reflective surface of the prism is guided, so that the inner diameter of the inner peripheral surface is measured”.
Further, JP-A-2011-196899 (Patent Literature 2) is disclosed. In this publication, disclosed is “each of laser displacement sensors is mounted which includes, at the base end side of an arm whose tip side is inserted into a tubular body and at each of three circumferential positions, a light-emitting part for emitting each laser light along the axial direction and toward the inside of the tubular body, and a light-receiving part for receiving each light reflecting from the inside of the tubular body. Each of prisms is circumferentially mounted at the tip side of the arm in the same way as each laser displacement sensor which turns laser light emitted from each light-emitting part at a right angle toward the bore surface of the tubular body laterally placed, and turns a part of the laser light directed laterally and then reflected from the bore surface of the tubular body toward the base end side of the arm. A calculation means is provided which obtains the reflection point of the laser light on the bore surface of the tubular body from a detection value of each laser displacement sensor, and calculates the bore of the tubular body from the three reflection points obtained”.